Variable inlet guide vanes (IGV) are used to introduce swirl into a compressor rotor to improve low speed operability as well as increase the flow capacity at high speeds. It is known to install radial IGVs directly upstream of the rotor, which provide a close-coupled direction of the flow into the rotor. An alternative is to use axial IGVs, i.e. having a pivot axis parallel to the engine axis; such may allow for a shorter engine, but usually provide a less optimum swirl profile at the rotor leading edge.
An acoustic condition called Vortex Whistle has been found to occur in compressors with axial IGVs located in radial inlets. As the air approaches a sonic condition, a loud pure tone noise is emitted. This usually occurs during ground or flight idle engine conditions with high IGV angles and is undesirable.